1. Field of the Invention
The present invention relates to a flexible printed board and, more particularly, to a flexible printed board on which a semiconductor element is flip-chip-bonded.
2. Description of the Related Art
In a conventional compact device such as a camera, a flexible printed board is frequently employed to arrange various electric circuits in a compact housing. A technical means for bonding an element such as an IC by a so-called flip chip bonding method is also known.
This flip chip bonding method is a bonding method for arranging an element such as an IC on a printed board upside down. According to this method, an external electrode on the IC is subjected to a bump (projection) process to directly connect the external electrode to the pattern of the board or to connect the external electrode to the pattern through conductive particles. As a concrete example of the flip chip bonding method, a bonding method is disclosed in Japanese Examined Patent Publication No. 62-6652, Japanese Unexamined Patent Publication No. 7-211423, and the like.
A general flexible printed board is formed such that a copper foil is interposed between sheets each having a thickness of several tens .mu.m. Since the sheet consists of an almost transparent material such as polyimide or polyester, the flexible printed board is considerably inferior to a hard printed board in light-shielding property.
When a semiconductor element such as an IC is bonded on a flexible printed board being inferior in light-shielding property by the flip-chip bonding method, the following drawbacks occur. More specifically, when such a flexible printed board is applied to a product such as a camera which is used under outside strong light or a product used to guide object light to the inside of the product, sunlight, illumination light, or leakage light from a built-in electronic flash, a date LED, or the like is incident on the surface of the semiconductor element such as an IC because the flexible printed board itself is inferior in light-shielding property as described above. As a result, in the semiconductor element, a change in performance or an erroneous operation is disadvantageously caused by a photo-electric effect.